The present invention relates to an amplifier of the type which can have its transmission factor controlled by a control voltage and which can be switched between two operating states having respectively complementary control characteristics. The transmission factor of an amplifier is its gain or attenuation factor.
Such amplifiers are required, for example, to suppress noise in compander systems, and amplifiers of this type are disclosed in German Offenlegungsschrift [Laid-Open Application] No. 22 18 823 and German Auslegeschrift [Published Application] No. 24 06 258 and corresponding U.S. Pat. No. 3,969,680. In these systems an amplifier is required which has a certain control characteristic determining the relation between the output/input signal ratio and a control voltage during dynamic compression, and which has a complementary control characteristic relative to the above relation during dynamic expansion.
German Offenlegungsschrift [Laid-Open Application] No. 22 18 823 teaches that the gain of an amplifier which can be switched between two operating states can be varied by means of a controllable impedance which, in one operating state of the amplifier, is connected to a negative feedback path of the amplifier and, in the other operating state, is connected to the input of the amplifier.
Due to a drawback which primarily exists in simple compander systems, namely the modulation of the noise components in dependence on low frequency useful signals, there is known to exist the requirement, as disclosed in Funktechnik, 1973, pages 55-57, particularly in FIG. 6, to shift the lower limit frequency of the circuit effecting the dynamic compression or expansion, respectively, toward higher frequency values in dependence on the amplitude of the useful signals, the shifted characteristic being as parallel as possible to the original.
In the amplifier disclosed in German Offenlegungsschrift [Laid-Open Application] No. 22 18 823, the control voltage for the controllable impedance is derived from the useful signal, and a capacitance is connected in series with the controllable impedance so that the lower limit frequency of the amplifier is not highly dependent on the amplitude of the useful signals. However, the shift of the lower limit frequency obtained in this manner is limited to a small range. Moreover, in this known circuit, the amplitude of the amplifier output signal is also changed in an undesirable manner in the transmission band.